A light-emitting diode (referred to hereinafter as LED) represents one of the most popular light-emitting devices today. In recent years, the luminous efficacy of LEDs, defined in lumens per Watt, has increased significantly from 20 lumens per Watt (approximately the luminous efficacy of an incandescent light bulb) to over 500 lumens per Watt, which greatly exceeds the luminous efficacy of a fluorescent light at 60 lumens per Watt. In other words, for a fixed amount of light output, LEDs consume approximately one sixth of the power compared to fluorescent lights, and almost negligibly small compared to incandescent light bulbs. Accordingly, it is not surprising today that lighting fixtures with LEDs have recently been replacing incandescent light bulbs and fluorescent light tubes. A new term “Solid-State Lighting” has been created. The term “Solid-State Lighting” refers to the type of lighting that uses semiconductor light-emitting diodes, such as an LED rather than traditional light sources.
In the field of solid-state lighting, most of the light sources are white light. For LEDs, white light may be obtained by a method referred to as “RGB white”. In this method, colored light emitted by LEDs, usually in primary colors such as red, green and blue light, is mixed to obtain white light. However, in some other solid-state lighting sources, white light may be obtained by other arrangements of one or more LEDs, which need not be colored LEDs.
Most light sources used in solid-state lighting may be further categorized by color temperature. The color temperature of a light source indicates the relative color appearance of the particular light source on a scale from “warmer” (more yellow/amber) to “cooler” (more blue) light. Color temperatures are generally given in Kelvin or K. Color temperatures over 5,000K are called cool colors (bluish white), while lower color temperatures (2,700-3,000 K) are called warm colors (yellowish white through red).
However, white light solid state light sources made from LEDs may be susceptible to process variation and other effects due to variation in manufacturing process. Color point of white light solid state light sources made from LEDs may be difficult to control and often the variation may be huge. The color point of the LEDs may vary substantially even using the same equipment and the same material. The variation may be to the extent that products produced at the same time using the same equipment are noticeably different in terms of color point and brightness.
Generally, one solution to the process variation issue may be by binning the products in accordance to the color temperature and the brightness of the LEDs so that products with similar brightness and color temperature can be separated and assembled together into lighting fixtures. The binning process may cause significant production yield loss, as the process variation may be huge. From lighting fixture manufacturer's perspective, the binning is not desirable. In order to fulfill the market needs of a wide range of color temperature ranging from warm white lighting fixtures to cool white lighting fixtures, lighting fixture manufacturers may have to manage significant inventories. For example, if the manufacturer uses 10 color bins, he may need to stock up to ten times inventories compared to ordinary manufacturing method without binning. The binning process may not be cost effective.